The present invention relates generally to rotary machines and, more particularly, to a high pressure first stage turbine.
At least some steam turbines have a defined steam path which includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. Steam leakage, either out of the steam path or into the steam path, from an area of higher pressure to an area of lower pressure may adversely affect an operating efficiency of the turbine. For example, steam-path leakage in the turbine between a rotating rotor shaft of the turbine and a circumferentially surrounding turbine casing, may lower the efficiency of the turbine.
To facilitate minimizing steam-path leakage to the atmosphere, at least some known steam turbines use a packing casing that includes a plurality of labyrinth seals. Some known labyrinth seals include longitudinally spaced-apart rows of labyrinth seal teeth which are used to seal against pressure differentials that may be present in the steam turbine. Seal members, such as brush seals or leaf seals, may also be used in an attempt to reduce leakage through a gap defined between two components.
In an attempt to reduce leakage of steam from the root of the stage into the end of the section, low reaction roots have been used. Generally, a low reaction root causes the leakage steam at the root to be at a reduced pressure as compared to the main flow pressure. Negative reaction and steam balance holes induce steam from the discharge of the stage through the steam balance holes to feed the leakage path. As a result, most of the steam that leaks has already been expanded through the stage and produced useful work. Such low reaction roots, however, may result in less efficient expansion of the steam than high reaction roots. A high degree of reaction facilitates ensuring there is a positive pressure on the rotating blade at all diameters, which in turn facilitates efficient expansion of the steam.
Another known approach is to use steam from a space between a nozzle and a bucket to feed the leakage path. The leakage path through the end section is decreased because the nozzle decreases the steam pressure, thus decreasing the source pressure for the leakage path. However, none of the steam used to feed the leakage path expands through the bucket to produce useful work. In addition, in-service variations of the reaction of the stage due to wear and tear may impact other machine parameters in a disadvantageous manner.
Yet another known approach utilizes high reaction stage roots with the leakage flow fed from the bowl of the stage. However, the bowl pressure is higher than the intermediate pressure at the roots. As a result, the source pressure for the leakage flow is higher and the leakage flow will increase, which leads to decreased efficiency.